CAPSAICIN has a specificity for nociceptive, or pain-transmitting, type C nerve fibers.

That, investigators say, is why it can block pain while not affecting sense of touch, pressure, or vibration.

And it is why this active agent in Tabasco sauce is becoming a common condiment in many basic science laboratories.

Topical application triggers a burst of the neuropeptide substance P from the C fibers, causing an initial burning sensation. Investigators say that unpleasant experience keeps some users from using it long enough to discover its full potential benefit.

Preventing Pain's ReturnThat benefit comes later because capsaicin also keeps substance P from being replenished, thus blocking further conduction of pain. In one study using 0.075% capsaicin ointment to treat diabetic neuropathy, it took 4 weeks for pain relief to become significant. (See accompanying article.)This delayed benefit is likely due to calcium-mediated structural changes, says James R. Brorson, MD, a